Soliton pulse transmission over long waveguide fiber lengths

ABSTRACT

Disclosed is an optical circuit for filtering and frequency modulation of soliton signal pulses traveling over long spans of waveguide fiber. The circuit makes use of the filtering properties of a non-linear optical loop mirror (NOLM). The time difference between control pulses and signal pulses co-propagating in the NOLM is controlled to increase or decrease the centroid shift of the signal pulses. The signal and control pulse streams are derived from a single stream of soliton pulses. The NOLM serves to filter low power noise from the soliton signal pulses at the same time as it shifts the centroid frequency of the soliton signal pulses up or down. The circuit can be inserted at advantageous points along a waveguide fiber transmission line to allow propagation of solitons, without electronic regeneration, over line lengths of 100 km.

This application is based upon the provisional application Ser. No.60/084,822, filed May 8, 1998, which we claim as the priority date ofthis application.

This invention was made with government support under DARPA contractnumber F-30602-97-1-2020.

BACKGROUND OF THE INVENTION

The invention is directed to a circuit for recovering the shape andspectrum of an optical signal after the signal has traversed a length ofoptical waveguide fiber. In particular, the circuit is used to preservethe shape of solitons over long lengths of waveguide fiber without useof electronic regeneration.

The value of soliton transmission of information over optical waveguidefibers is recognized in the art. The possibility of essentiallydispersion-free transmission of pulses over long fiber lengths withoutelectronic regeneration has encouraged work in the area of maintainingsoliton signal integrity in extended transmission lines. With theintroduction of waveguide fibers having attenuation in the range of afew tenths of a decibel per kilometer and optical amplifiers, solitonshave become even more attractive as the transmission method of choice invery high bit rate systems or those that make use of wavelength divisionmultiplexing.

A problem to be addressed in using soliton signals is how to controlchanges in the soliton time window, often called timing jitter, to avoidoverlap with neighboring pulses. In addition, one should provide forfiltering of noise that arises from the energy shedding of the solitonpulse as it undergoes shape changes during propagation along the fiber.Noise also originates in amplified spontaneous emission in the opticalamplifiers

At bit rates >50 Gb/sec, the soliton pulse width must be less than about5 ps to avoid overlap between adjacent soliton time windows. This pulsewidth is in general sufficient to minimize errors at the receiver end ofthe transmission. At the same time, the soliton period for these shortpulse widths must be kept short relative to the preferred opticalamplifier spacing, about 25 km. Thus there is a need to remove theenergy lost by the soliton so that signal to noise ratios are at adesired level, inter-symbol interference is eliminated, and opticalamplifiers do not become saturated by presence of noise signals.

Another consideration is the self-frequency shift of a narrow time widthsoliton due to differential Raman amplification of the pulse wavelengthspectrum. This shift should be compensated in order to maintain thesoliton wavelength within the desired low attenuation operating windowand within the gain spectrum of the optical amplifiers.

The control of soliton timing jitter using a non-linear optical loopmirror (NOLM) is described in U.S. Pat. No. 5,757,529, Desurvire et al('529). In the '529 patent, a loop mirror is used as a switch thatrejects system noise by preferential switching of the soliton signalthrough the NOLM. The switching is brought about my means of a stream ofcontrol pulses introduced into the NOLM. The overlap of the signalpulses and the control pulses in time determines the switchingcharacteristics of the NOLM. Because relative timing difference betweenthe signal pulse and control pulse are critical, the '529 patentproposes a clock extraction or recovery circuit that produces a clocksignal from the signal solitons. Such clock recovery circuits addconsiderable cost and complication to an optical circuit employing aNOLM. In some clock circuits, electo-optical devices are employed.

Thus, there is a need for a relatively simple and low cost means forrecovering the shape of soliton signal after it has traversed a span ofabout 25 km of waveguide fiber. Also there is a need to address theproblem of selffrequency shifting of the soliton signals without resortto elaborate, expensive optical or electo-optical circuits. Theinvention disclosed and described in this application incorporatessimplicity and low cost into an optical circuit including a NOLM thatsimultaneously removes transmission circuit noise and recovers theoriginal spectrum of the soliton signals.

SUMMARY

The present invention is an optical circuit for noise filtering andfrequency modulation of soliton pulses. The circuit includes a NOLMhaving its ends optically joined to the output ports of an NXN or firstcoupler, where N is at least two. Signal pulses are optically coupled toone of the input ports of the coupler and the signal pulses are thusdivided into a clockwise (CW) and a counter-clockwise (CCW) stream ofpulses propagating in the NOLM. A tap coupler is optically coupled to apoint along the length of the NOLM. A stream of control pulses are inputinto the tap coupler which then couples the control pulses to the NOLM.Both the signal pulses and the control pulses can originate from asingle optical soliton source. A source optically couples a stream ofsolitons to the input port of a splitting coupler which divides thestream of solitons into a stream of control pulses and a stream ofsignal pulses. Depending upon the direction along the loop mirror inwhich the tap coupler couples the control pulses, the control pulseswill co-propagate with either the CW or CCW propagating signal pulses.The optical circuit is symmetric in the sense that the circuit may beconfigured to cause interaction between the control pulses and eitherthe CW or CCW propagating signal pulses. It will therefore be understoodthat the description of the circuit given herein applies equally to theinteraction of CW or CCW propagating signal pulses and the controlpulses.

The optical path between the splitting coupler and the tap coupler is anoptical fiber optically connected between the two couplers. So too, theoptical path between the splitting coupler and the input of the firstcoupler is an optical fiber. The amount of interaction between thecontrol pulses and the signal pulses depends upon the amount of overlapof the two sets of pulses as they travel through the optical loopmirror. This amount of overlap is controlled by selecting the lengthsother two fibers that connect the respective pairs of coupler ports toprovide a pre-selected lead or lag time of the signal pulses relative tothe control pulses.

The interaction of control pulses with co-propagating signal pulsesproduces both a shift in centroid wavelength and phase (relative to thecounterpropagating signal pulses) of the signal pulses. The centroidwavelength may be shifted up or down depending upon whether the controlpulses lead or lag the co-propagating signal pulses. The amount of phaseshift of the co-propagating signal pulse depends upon the magnitude ofthe lead or lag time between the control and signal pulses.

The selection of the lengths of the connecting waveguide fibers thusallows one to select the amount of centroid and phase shift of theco-propagating signal pulses in the NOLM.

In addition, the NOLM reflects the stray energy waveforms (noise), dueto power shedding of the solitons or due to amplified spontaneousemission. Because the amplitude of the stray pulses is below the levelat which nonlinear phase shifting occurs, no phase shift occurs betweenthe CW and CCW propagating noise so they are not switched through thefirst coupler.

Thus the optical circuit disclosed and described herein serves to shiftthe centroid wavelength of the co-propagating signal pulses, shift thephase of the co-propagating signal pulse to switch the signal pulsesthought the first coupler, and also to remove the low amplitude noiseaccumulated in the optical circuit. The shift in centroid can be chosento offset any centroid shift of the solitons caused by differentialRaman gain across the soliton spectrum. These functions are accomplishedwithout use of clock extraction circuits or synchronized control pulsesfrom a source separate from the signal soliton source.

In one embodiment of the optical circuit, the lead or lag time of thecontrol pulses relative to the co-propagating signal pulses is withinthe range of about three times _(T). Here _(T) is the soliton pulsewidth expressed as a time interval between the half maximum power pointsof the soliton.

An embodiment of the invention includes polarization adjustingcomponents in one or both of the waveguides joined to the tap and firstcoupler. The control pulse is given a pre-selected polarization relativeto the polarization of the signal pulse so that the control pulse mayinteract with the signal pulse and then be conveniently removed from theloop mirror fiber using a polarization selective coupler. The relativepolarization of the respective control and signal pulses is such thatthe interaction between control pulses and the co-propagating signalpulses is effective to produce the desired phase shift and centroidshift of the signal pulses. Design alternatives that fall within thescope of this invention include relative polarization of control ascompared to signal pulses that ranges from orthogonal to parallel. Inthose cases where the control and signal pulse axes are parallel ornearly so, other means of filtering the control pulse from the NOLM areavailable. Such means include gratings and wavelength or amplitudediscriminators.

In an alternative embodiment, the tap coupler is polarization selective,to provide polarized control pulses in the optical loop mirror. Afterinteraction with the co-propagating signal pulses, the control pulsesmay then be coupled out of the optical loop mirror by means of a secondpolarization selective tap coupler located farther along the opticalloop mirror in the direction of travel of the control pulses.

In yet another embodiment of the invention, a band pass filter isinserted in the optical path, e.g., an optical fiber, of the signalpulses that have passed through the optical loop mirror. An advantageousfeature of this configuration is that the center wavelength of the bandpass filter can chosen to coincide with the centroid of the signalpulses exiting the NOLM. Because the NOLM as used herein shifts thecentroid, a circuit can be designed which passes the centroid shiftedsolitons while rejecting noise near the wavelength of the originalsoliton. Thus the signal pulses pass through the band pass filter withminimum loss of power while noise pulses at the wavelength of thecontrol pulses or original signal pulses are reflected or absorbed. Thisis an alternative use of the optical circuit as compared to usedescribed above where the optical circuit including a NOLM is used tocancel self frequency shifting of the soliton due to differential Ramangain. Signal pulse centroid shifts in the range of +/−2 nm are possibleso that the offset of the center wavelength of the band pass filter iseffective to filter soliton pulses or noise at the wavelength of thesource solitons.

In yet a further embodiment, an optical amplifier can be opticallyincorporated into the fiber that carries the signal pulses switchedthrough the optical loop mirror. This amplifier serves to offset lossesin the loop mirror and in the band pass filters. The amplifiers can be,for example, lumped or distributed erbium doped optical amplifiers. Alsosemiconductor optical amplifiers may be used.

The length of the waveguide fiber of the optical loop mirror itself isselected to be effective to allow a desired interaction length betweenthe control and signal pulses. A non-linear optical loop mirror lengththus is in the range of about 200 m to 2 km. For signal pulses in thewavelength range around 1550 nm, a dispersion shifted fiber, a fiberhaving a dispersion zero in the range of about 1400 nm to 1600 nm, isused in the NOLM. Longer and shorter optical loop mirror lengths havebeen used by those skilled in the art. These longer or shorter lengthscould be incorporated into the optical loop mirror of the presentinvention.

In a further embodiment of the invention, the second waveguide fiber (orthe first waveguide fiber) can include a variable time delay componentso that the relative lag or lead time between signal and control pulsesmay be adjusted.

Additional embodiments of the invention include those that havedifferent ratios of coupling to the output ports. For example, the firstcoupler can divide the power equally between the CW and CCW pulsespropagating in the optical loop mirror. Advantageous configurationsinclude those having the coupling ratios of the first coupler in therange of about 30%::70% to 70%::30%. Likewise the power ratios of thesplitting coupler outputs can be varied between about 10%::90% to90%::10%.

In a particular embodiment of the invention, the splitting couplerprovides about 10% of the input soliton power to the tap coupler. Thisconfiguration provides-for higher power signal pulses exiting theoptical loop mirror. An optical amplifier can be added into the secondfiber, which optically joins the splitting coupler to the tap coupler,to maintain the control pulse amplitude at a a level sufficient toprovide for non-linear cross phase modulation of the co-propagatingsignal pulse by the control pulse.

The optical amplifier in this second connecting fiber can be a lumped ordistributed fiber amplifier or a semiconductor optical amplifier. Inthis latter case, the semiconductor optical amplifier can be selected toshift the wavelength of the control pulses. The tap couplers thatrespectively couple the control pulses into and out of the loop mirrorcan then be chosen to be wavelength selective. This is yet anotheralternative to the use of polarization selective tap couplers.

The optical circuit described above can be used in long transmissionlinks designed for soliton signals. Thus an additional aspect of theinvention is an optical waveguide transmission circuit that incorporatesone or more of the optical circuits including a NOLM described above.The incorporation of one or more of these optical circuits into atransmission circuit serves to maintain the desired shape and signal tonoise ratio of the signal solitons without recourse to electronicregeneration of the signal.

In an embodiment of the optical waveguide transmission circuit, anoptical loop mirror circuit is inserted after the solitons havetraversed between 25 km to 50 km of waveguide fiber. Signals exitingthis first optical circuit can traverse about 50 km to 75 km of thetransmission circuit before another optical loop mirror is needed toagain reshape the soliton signal pulses.

In a further embodiment of the optical waveguide transmission circuit, anumber of optical amplifiers or band pass filters may be opticallyincorporated into the extended fiber lengths before or after the opticalloop mirror circuits.

Additional features and advantages of the invention will be set forth inthe detailed description which follows, and in part will be readilyapparent to those skilled in the art from that description or recognizedby practicing the invention as described herein, including the detaileddescription which follows, the claims, as well as the appended drawings.

It is to be understood that both the foregoing general description andthe following detailed description are merely exemplary of theinvention, and are intended to provide an overview or framework forunderstanding the nature and character of the invention as it isclaimed. The accompanying drawings are included to provide a furtherunderstanding of the invention, and are incorporated in and constitute apart of this specification. The drawings illustrate various embodimentsof the invention, and together with the description serve to explain theprinciples and operation of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic drawing of a non-linear optical loop mirror(NOLM).

FIG. 2 is a schematic drawing of an optical circuit including a NOLM.

FIG. 3 is a chart showing exemplary wavelength shift of a signal pulsevs. the time delay of the control pulse.

FIG. 4 is a schematic drawing of an optical waveguide transmissioncircuit using two optical circuits each including a NOLM.

FIG. 5 is an auto-correlation chart of an input signal pulse compared toan output signal pulse that has traveled through an optical transmissioncircuit including 75 km of fiber and two optical circuits each having aNOLM.

FIG. 6 is a chart of the soliton pulse intensity vs. wavelength for aninput signal pulse compared to output signal pulse that has traveledthrough an optical transmission circuit including 75 km of fiber and twooptical circuits each having a NOLM.

FIG. 7 is a schematic drawing of an optical waveguide transmissioncircuit using two optical circuits each including a NOLM.

FIG. 8 is an auto-correlation chart of an input signal pulse compared tooutput signal pulses that have traveled through 100 km lengths of atransmission circuit having and not having one or more optical circuitseach including a NOLM.

FIG. 9 is a chart of the soliton pulse intensity vs. wavelength for aninput signal pulse compared to output signal pulses that have traveledthrough 100 km lengths of a transmission circuit having and not havingone or more optical circuits each including a NOLM.

DETAILED DESCRIPTION OF THE INVENTION

Reference will now be made in detail to the present preferredembodiments of the invention, examples of which are illustrated in theaccompanying drawings. Wherever possible, the same reference numberswill be used throughout the drawings to refer to the same or like parts.

A basic schematic drawing of the optical circuit including a NOLM isshown in FIG. 1. The ends of non-linear fiber loop 2 are opticallyjoined to coupler 4. Via fiber 10, the signal pulses arrive at coupler 4and are coupled into the loop 2 of the loop mirror as a clockwise (CW)and a counter-clockwise (CCW) stream of pulses propagating in loop 2.Control pulses are coupled into the loop 2 through coupler 8, which inthis embodiment incorporates any of the alternative selective methodsdescribed above including polarization, amplitude or wavelengthselective coupler so that the control pulses propagating in loop 2 havea particular property that distinguishes them from the signal pulses.The distinction is sufficient to allow the control pulses to beselectively coupled out of the loop mirror. Yet the distinction betweencontrol and signal pulses is not so great as to preclude the desiredinteraction of the signal and control pulses. For the case shown in FIG.1, the control pulses interact with the CW propagating signal pulses.The time delay between the signal and control pulses is selected toproduce the desired phase and centroid shifts of the signal pulses. Theinput signal pulse is illustrated by curve 16 in FIG. 1. A comparison ofthe input signal pulse 16 with the output signal pulse 18 illustratesthe case in which the control pulse lags the signal pulse and sodecreases the centroid wavelength. Thus pulse 18 has centroid wavelengthλ₁′ which is less than λ₁, the centroid wavelength of the input signalpulse 16. The term centroid is used here to describe the first moment ofan amplitude versus wavelength chart representative of the pulse. Theuse of this definition is preferred over defining the pulse by means ofits peak amplitude wavelength. The centroid definition takes intoaccount non-uniformity in the pulse shape.

The interaction of control pulses with the CW signal pulses, through thenon-linear effect, cross phase modulation, serves to change the phase ofthe CW pulses relative to CCW pulses, so that the CW and CCW have atleast a partial phase match at coupler 4. Constructive interference thusoccurs at coupler 4 and the signal pulses are switched out of the NOLMinto fiber 12. Without the phase change induced in the signal pulses bythe control pulses only destructive interference would occur at coupler4 and no switching would occur. Although a 180° phase shift of the CW ascompared to the CCW pulses provides for maximum signal pulse powerswitching, switching of a portion of the power will still occur atcoupler 4. The remaining control pulse power is coupled out of loop 2 bycoupler 6 to prevent unwanted interaction of control pulses with CW andCCW signal pulses at coupler 4. Coupler 6 may be chosen to bepolarization, amplitude, or wavelength sensitive.

The switching and centroid shifting function of the NOLM can bedescribed as follows. An attractive force is present between solitonsclose to one another in an optical path. This attractive force shiftsthe spectral power distribution of the control and signal pulses,thereby shifting their respective centroids. If the control pulse leadsthe signal pulse, the centroid (center wavelength as defined above) ofthe signal soliton will be increased. Conversely, a lagging controlpulse will decrease the centroid wavelength of the signal pulse. Theamount of increase or decrease of signal pulse centroid depends upon themagnitude of the difference in travel time of the signal and controlpulses. The spectral redistribution of signal pulse power is manifestedas a phase shift of the CW signal pulses relative to the CCW signalpulses. Although this simplified explanation is useful in understandingthe invention, the correctness of this explanation in no manner affectsthe scope of the invention as claimed.

In addition to the switching and phase shifting functions, the NOLM alsoserves to reflect the noise that may be traveling with the signalpulses. In coupler 4, the noise is split into CW and CCW propagatingportions. Because the noise is typically below the amplitude thresholdof non-linear phase shifting in the loop fiber 2, the CW and CCW arriveback at coupler 4 with no relative phase shift and thus are not switchedto output fiber 12.

The optical circuit including a NOLM is shown schematically in FIG. 2. Asoliton source (not shown) supplies a pulse stream to splitting couple20, which divides the pulse stream into control and signal pulsestreams. Polarization adjusting devices 22 (for example, a combinationof quarter wave plate and a half wave plate or a combination of aquarter, a half, and another quarter wave plate) are placed in therespective control pulse paths and signal pulse paths. The polarizationadjusting devices 22 in the fiber loop 2 are used to maximize efficiencyof control and signal pulse interaction in the case in which the pulsepolarization of control and signal pulses is parallel. In the case oforthogonally polarized pulses, devices 22 blocks leakage of light energyhaving other than orthogonal polarization. These polarization adjustingdevices are tuned to provide for maximum signal power switching out ofthe NOLM. An optical amplifier 24 is optically joined in the controlpulse waveguide fiber path to ensure that the amplitude of the controlpulse is in the non-linear regime of loop fiber 2. The use of amplifier24 is preferred because it allows one to choose a splitter coupler 20which diverts a larger amount of the source power to the signal pulse.The overlap of signal and control pulses is set by the relative traveltime of the respective control and signal pulses from point A, at theoutput of coupler 20, to point B on loop 2 where the control pulse iscoupled into loop 2. The overlap may be set by adjusting the lengths ofwaveguide fibers that optically join the components in the respectivesignal and control pulse paths. By means of this relatively simpleoptical circuit, the signal is selectively switched through the NOLM,the stray noise energy from power shedding of the solitons orspontaneous emission in the amplifier is reflected, and the centroid isshifted by a pre-selected amount.

The amplifier 24 may be a lumped or distributed fiber amplifier or asemiconductor optical amplifier. As discussed above, the semiconductoroptical amplifier can advantageously be used to shift the wavelength ofthe control pulses so that a wavelength selective second tap coupler maybe used to remove the control signal from the loop. The use of awavelength discriminating tap couple allows one to adjust thepolarization of the control pulses to be parallel or nearly parallel tothat of the signal pulses.

The optical circuit of FIG. 2 is afforded greater flexibility byintroducing variable time delay component 26 into the optical path ofthe control signal. Optical time delay components are known in the artand will not be discussed here. By means of variable delay 26, the lagor lead of the control pulse can be adjusted, which in turn adjusted theamount of overlap of the signal and control pulses in loop 2. Thus, theamount of centroid shift and phase shift of the signal pulse can beadjusted. For example, one may adjust the variable delay (or the fiberpath length) to cancel self frequency shifting due to Raman differentialamplification.

Dotted curve 28 of FIG. 3 shows the experimental signal pulse centroidshift as delay is changed. Solid line 29 is a theoretical curve derivedfrom a model which assumes a background birefringence of 5×10⁻⁷. At adelay of zero, i.e., the signal and control pulses completely overlap,there is no centroid movement. Maximum signal pulse centroid movement isachieved near time delays of about +/−1.7 ps. The amount of centroidshift is in the range of about +/−2 nm, which will allow a signal soshifted to be filtered by means of a narrow band pass device.

EXAMPLE 1

An optical transmission circuit was assembled in accord with FIG. 4.Ring laser 30 produced narrow pulses (470 fs) at 1560 nm which werebroadened to about 1.5 ps pulses by passing them through a 2 nm gaussianband pass filter 32. The power output after the first optical amplifier24 of each span was about 275 μW, corresponding to solitons of orderN=1.1. The pulses then passed through 25 km of step index fiber 34 andoptical isolator 38. The splitting coupler 20 divided the input power ata 75::25 ratio of control pulse power to signal pulse power,respectively. The coupler 4 divided the signal power equally between CWand CCW pulse streams in first NOLM 36 to maximize the reflection ofnoise signal power in the NOLM. The delay time between signal andcontrol pulses was selected to cancel the soliton self frequency shiftpicked up in the first 25 km of waveguide fiber. The signal wastransmitted through another isolator 38, optical amplifier 24, 25 km offiber 34, an additional isolator 38 and into the second NOLM 36. Thecouplers of the second NOLM were configured as for the first NOLM. Thesignal pulse was switched through the second NOLM and, as describedabove, passed through an optical amplifier, 25 km of fiber and a pair ofisolators. The signal was then amplified a final time by fourth opticalamplifier 24 and sent to a receiver for detection and measurement.

The results of the measurements are shown in FIGS. 5 and 6. Theauto-correlation function in FIG. 5 shows a comparison of the inputpulse, detected after first filter 32 in FIG. 4, curve 42 as compared tothe signal pulse exiting the second NOLM, curve 44. The soliton shapewas essentially completely recovered by the optical circuits including aNOLM. Curve 46 is the signal pulse measured after its transit throughthe full 75 km of the transmission circuit. Pulse 46 shows essentiallyno noise signal and about a threefold broadening of the pulse. Incontrast, curve 48 shows the pulse detected after the signal hadtraversed the entire 75 km transmission circuit, but with the twooptical circuits including a NOLM removed. The noise floor of curve 48is raise by more than a factor of ten and the width is quite broadcompared to input pulse 42 or 75 km pulse 46. The chart shows the markedimprovement in pulse shape provided by the optical circuit including aNOLM.

The chart of pulse spectrum in FIG. 6, also shows the efficiency of theNOLM in canceling soliton self frequency shifting. The input pulse isshown as curve 50. The signal pulse spectrum detected after the signalpulse had exited the first NOLM 36 is shown as curve 52. The spectralshift is very small. Likewise, curve 54 shows the signal spectrum afterthe signal pulse has traversed the full 75 km transmission circuit ofFIG. 4. The centroids of respective curves 50, 52, and 54 are seen to benearly identical. Curve 56 shows the signal pulse spectrum for a signalwhich has been transmitted through the full 75 km transmission circuitwith the optical circuits, each including a NOLM, removed. The centroidof the spectrum is shifted by several nm and the spectrum shape has beenmarkedly altered.

EXAMPLE 2

A second optical transmission circuit was assembled as shownschematically in FIG. 7. The circuit differs from that of FIG. 4 in thatthe first optical circuit including a NOLM 36 was placed after thesignal had been transmitted through 50 km of single mode waveguidefiber. The output signal from the first NOLM 36 was then transmittedthrough an additional 50 km of single mode fiber before the signal wasagain conditioned by and switched through an optical circuit including aNOLM located at the end of the 100 km transmission circuit. Variableattenuators 40 were introduced into the circuit after each amplifier toadjust the soliton amplitude to the desired value.

The chart of the auto-correlation shows that the optical circuits eachincluding a NOLM served to almost completely recover the shape and widthof the input signal pulse. In FIG. 8, compare the input pulse 58 to theoutput pulse 60 which was transmitted through 100 km of single modefiber.

The intensity versus wavelength chart of FIG. 9 shows that the spectrumof the input pulse 62 has also been almost completely recovered inoutput pulse spectrum 64.

Thus the optical circuit including a NOLM provides, improvement ofsignal to noise ratio, a shift of soliton centroid wavelength tocompensate self frequency shift, and recovery of the soliton pulsewidth. The circuit is relatively simple yet still provides forpicosecond soliton transmission over 100 km transmission links.

It will be apparent to those skilled in the art that variousmodifications and variations of the present invention can be madewithout departing from the spirit and scope of the invention. Thus, itis intended that the present invention include the modifications andvariations of this invention provided they come within the scope of theappended claims and their equivalents.

We claim:
 1. An optical circuit for filtering and frequency modulationof solitons propagating in the circuit comprising: anon-linear-optical-loop mirror having ends and a length; a first couplerhaving at least two input ports and two output ports, wherein the endsof the optical loop mirror are each optically joined to a respectiveoutput port of the first coupler; a tap coupler having at least oneinput port and an output port optically coupled to a point along theoptical loop mirror length; a splitting coupler having at least oneinput port and two output ports, a first splitting coupler output portoptically joined to a first input port of the first coupler by a firstoptical fiber and a second splitting coupler output port joined to theinput of the tap coupler by a second optical fiber; wherein, therespective lengths of the first and second optical fibers are chosen toprovide a pre-selected difference in travel time of solitons in thecircuit between the respective splitter coupler output ports and saidpoint along the optical loop mirror at which the tap coupler isoptically coupled to the optical loop mirror; and, wherein thedifference in travel time is small enough to provide for overlap in theoptical loop mirror of the solitons coupled into the optical loop mirrorfrom the output port of the tap coupler and the solitons, whichco-propagate with the solitons coupled by the tap coupler, coupled intothe optical loop mirror from the first input port of the first coupler.2. The circuit of claim 1 wherein the difference in travel time betweenthe co-propagating solitons is no greater than about three times _(T),wherein _(T) is the soliton pulse width expressed as a time intervalbetween the half maximum power points of the soliton.
 3. The circuit ofclaim 1 wherein the tap coupler is a first polarization selective tapcoupler that couples solitons of a particular polarization into theoptical loop mirror.
 4. The apparatus of claim 3 further comprising asecond polarization selective tap coupler, optically coupled to theoptical loop mirror to remove polarized solitons coupled into the loopmirror by the first polarization selective tap coupler, wherein thecoupling point of the second polarization selective tap coupler with theoptical loop mirror is farther along the length of the optical loopmirror, relative to the coupling point of the first polarizationselective tap coupler, in the direction of propagation of the coupledpolarized solitons.
 5. The circuit of claim 4 further comprising twopolarization adjusting components optically incorporated into therespective first and second waveguide fibers, wherein soliton pulsespassing through the respective polarization adjusting components havepolarization axes that form a pre-selected included angle therebetween.6. The circuit of claim 5 wherein the included angle is 90°.
 7. Thecircuit of claim 5 wherein the included angle is zero.
 8. The circuit ofclaim 1 further comprising an optical band pass filter optically joinedto the second input port of the first coupler by a third waveguidefiber, wherein the center wavelength of the pass band of the opticalfilter is offset by a pre-selected amount from the center wavelength ofthe solitons leaving the splitting coupler.
 9. The circuit of claim 8wherein the pre-selected offset of the center wavelength of the opticalband pass filter is in the range of about +/−2 nm.
 10. The circuit ofclaim 8 further comprising optical amplifying means opticallyincorporated into the third waveguide fiber.
 11. The circuit of claim 10wherein the optical amplifying means is selected from the groupconsisting of a lumped erbium doped optical amplifier, a distributederbium doped optical amplifier, and a semiconductor optical amplifier.12. The circuit of claim 1 wherein the time difference in travel timebetween the solitons directed to the input port of the tap coupler andthe solitons directed to the first input port of the first coupleroutput is positive.
 13. The circuit of claim 1 wherein the timedifference in travel time between the solitons directed to the inputport of the tap coupler and the solitons directed to the first inputport of the first coupler output is negative.
 14. The circuit of eitherone of claim 12 or 13 wherein the length of waveguide fiber included inthe optical loop mirror is in the range of about 200 m to 2 km and thewaveguide fiber has zero dispersion wavelength in the range of 1400 nmto 1600 nm.
 15. The circuit of any one of claims 1-13 further includinga variable time delay component optically incorporated into the first orsecond waveguide fiber.
 16. The circuit of claim 15 wherein the splittercoupler is selected to split the input power of the solitons between thefirst waveguide fiber and the second waveguide fiber in a ratio range ofabout 10%::90% to 90%::10%.
 17. The circuit of claim 15 wherein thefirst coupler divides the power of the solitons transmitted to therespective ends of the optical loop mirror in a ratio in the range ofabout 30%::70% to 70%::30%.
 18. The circuit of claim 1 further includingan optical amplifier optically incorporated into the second waveguidefiber.
 19. The circuit of claim 18 wherein the optical amplifier isselected from the group consisting of a lumped erbium doped opticalamplifier, a distributed erbium doped optical amplifier, and asemiconductor optical amplifier.
 20. The circuit of claim 19 wherein theoptical amplifier is a semiconductor optical amplifier that shifts thewavelength of the solitons entering the tap coupler relative to thewavelength of the solitons entering the first input port of the firstcoupler, and wherein the circuit further included a second tap coupler,that is wavelength selective, optically coupled to the optical loopmirror to remove wavelength shifted solitons coupled by the first tapcoupler, wherein the coupling point of the second tap coupler with theoptical loop mirror is farther along the length of the optical loopmirror, relative to the coupling point of the first tap coupler, in thedirection of propagation of the coupled wavelength shifted solitons. 21.The circuit of claim 1 further including polarization adjusting deviceswithin the non-linear-optical-loop mirror.
 22. An optical waveguidecircuit for transmission of soliton pulses comprising: a first circuitin accordance with any one of claim 1-13 or 18-20 wherein the input portof the splitting coupler is optically coupled to an extended waveguidefiber length that is carrying solitons; a second circuit in accordancewith any one of claim 1-13 or 18-20 wherein the input port of thesplitting coupler of the second circuit is optically joined to thesecond input port of the first coupler of the first circuit by a secondextended length of waveguide fiber.
 23. The optical waveguide circuit ifclaim 22 wherein the first extended waveguide fiber length is in therange of about 25 km to 50 km.
 24. The optical waveguide circuit ofclaim 22 wherein the second extended waveguide fiber length is in therange of about 50 km to 75 km.
 25. The optical waveguide circuit ofclaim 23 wherein either the first or second extended waveguide fiberlength includes at least one optical amplifier and at least one opticalband pass filter.
 26. The optical waveguide circuit of claim 24 whereineither the first or second extended waveguide fiber length includes atleast one optical amplifier and at least one optical band pass filter.